Power transistor including a sense emitter and a reference emitter for enabling power dissipation to be limited to less than a destructive level

ABSTRACT

A power transistor in which power dissipation may be limited to less than a destructive level is disclosed. The power transistor includes a base; a power emitter in the base; a sense emitter positioned in the base sufficiently close to the power emitter for enabling the temperature of the power emitter to be indicated as a function of the voltage across the junction of the base and the sense emitter; and a reference emitter positioned in a base at a remote position that would not be heated by the heat dissipated by the power emitter for enabling the temperature at the remote position to be indicated as a function of voltage across the junction of such base and the reference emitter. A voltage difference proportional to the temperature gradient in the power emitter induced by power dissipated in the power emitter can be sensed between the sense emitter and the reference emitter. Power dissipation in the power transistor may be limited in response to the sensed voltage difference.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to applicant's co-pending applicationentitled "System for Limiting Power Dissipation in a Power Transistor toLess Than a Destructive Level" filed on Sept. 16, 1977, application Ser.No. 833,736.

BACKGROUND OF THE INVENTION

The present invention generally pertains to a system for limiting powerdissipation in a power transistor to less than a destructive level.

Destructive secondary breakdown in a power transistor occurs when thepower being dissipated by the power emitter causes the temperature ofthe emitter to rise to a destructive level. The destructive powerdissipation level in a power transistor is not easily calculated and/ormay vary considerably between power transistors even though they havethe same nominal power rating. To prevent destruction of powertransistors it has been necessary to rate the operation of powertransistors considerably below their capabilities.

SUMMARY OF THE INVENTION

The present invention provides a power transistor having a structurethat enables it to be used in a system for limiting the powerdissipation of the power transistor to less than a destructive level.The power transistor of the present invention includes a base; a poweremitter in the base; a sense emitter positioned in the base sufficientlyclose to the power emitter for enabling the temperature of the poweremitter to be indicated as a function of the voltage across the junctionof the base and the sense emitter; and a reference emitter positioned ina base at a remote position that would not be heated by the heatdissipated by the power emitter for enabling the temperature at theremote position to be indicated as a function of voltage across thejunction of such base and the reference emitter. Accordingly, the powerdissipation of the power transistor can be limited to less than adestructive level in response to sensing a voltage differenceproportional to the temperature gradient between the remote position andthe power emitter induced by power dissipated in the power emitter. Thisvoltage difference can be sensed between the sense emitter and thereference emitter. The reference emitter is positioned in either thesame base as the power emitter or in a second base of the powertransistor.

Additional features of the present invention are discussed in thedescription of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of one preferred embodiment of apower transistor according to the present invention.

FIG. 2 is a schematic representation of an alternative preferredembodiment of a power transistor according to the present invention.

FIG. 3 is a schematic representation of another preferred embodiment ofa power transistor according to the present invention.

FIG. 4 is a schematic representation of still another preferredembodiment of a power transistor according to the present invention.

FIG. 5 is a schematic representation of yet another preferred embodimentof a power transistor according to the present invention.

FIG. 6 is a schematic circuit diagram of a system for limiting powerdissipation in power transistor of the present invention to less than adestructive level.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 through 5 inclusive schematically illustrate various embodimentsof power transistors in accordance with the present invention. The powertransistor of FIG. 1, includes a collector 12, a base 14 in thecollector 12 and a power emitter 16 in the base 14. For the sake ofclarity of illustration, conductors to collector, base and emitterterminals and the contacts by which such conductors are joined to thecollector, base and emitter are not shown in the drawing.

The power transistor of FIG. 1 further includes a sense emitter 18 and areference emitter 20, both in the base 14. The sense emitter 18 isdistributed around the power emitter 16 and is sufficiently close to thepower emitter 16 to enable the highest instantaneous temperature at anypart of the periphery of the power emitter 16 to be indicated as afunction of the voltage across the junction of the base 14 and the senseemitter 18. The reference emitter 20 is positioned in the base 14 at aremote position 23 that would not be heated by the heat dissipated bythe power emitter 16 for enabling the temperature at the remote position23 to be indicated as a function of the voltage across the junction ofthe base 14 and the reference emitter 20.

The power transistor of FIG. 2 is constructed in the same manner as thepower transistor of FIG. 1, except that the reference emitter 20 is in asecond base 22 that is separate from the base 14. Both the base 14 andthe second base 22 are in the collector 12. The reference emitter 20 ispositioned in the second base 22 at a remote position 23 that would notbe heated by the heat dissipated by the power emitter 16 for enablingthe temperature at the remote position 23 to be indicated as a functionof the voltage across the junction of the second base 22 and thereference emitter 20.

The power transistor of FIG. 3 is constructed in the same manner as thepower transistor of FIG. 1, except for the provision of a sense emitter24 instead of the sense emitter 18. The sense emitter 24 is positionedadjacent the power emitter 16. The sense emitter 24 is sufficientlyclose to the power emitter 16 to enable the temperature of the poweremitter 16 to be indicated as a function of the voltage across thejunction of the base 14 and the sense emitter 24.

The power transistor of FIG. 4 differs from the power transistor of FIG.1 in that a plurality of power emitters 26 are contained in the base 14and a sense emitter 28 is distributed around and between the poweremitters 26. The sense emitter 28 is positioned sufficiently close toeach of the power emitters 26 to enable the highest instantaneoustemperature at any part of the peripheries of the power emitters 26 tobe indicated as a function of the voltage across the junction of thebase 14 and the sense emitter 28. The power emitters 26 are connected toa common power emitter terminal (not shown).

The transistor of FIG. 5 differs from the power transistor of FIG. 4 inthe same manner that FIG. 2 differs from FIG. 1. The reference emitter20 is in a second base 22, which is separate from the base 14.

A system for limiting power dissipation in a power transistor accordingto the present invention to less than a destructive level is shown inFIG. 6. This system utilizes a power transistor 30. The power transistor30 includes a power emitter e_(p), a sense emitter e_(s) and a referenceemitter e_(r). The power emitter e_(p) has a large power dissipation;and the sense emitter e_(s) and the reference emitter e_(r) both areoperated at low power.

By sensing the voltage difference between the sense emitter e_(s) andthe reference emitter e_(r), an indication is provided of thetemperature gradient between the remote position 23 and the poweremitter e_(p) induced by the power dissipated in the power emittere_(p). The power emitter e_(p), the sense emitter e_(s) and thereference emitter e_(r) are matched for emitter-base voltage-temperaturecharacteristics as a consequency of being manufactured at the same time.Therefore with equal current densities in the sense emitter e_(s) andthe reference emitter e_(r), the voltage difference between the senseemitter e_(s) and the reference emitter e_(r) is zero volts when nopower is being dissipated in the power emitter e_(p). However, oncepower is dissipated in the power emitter e_(p), both the power emittere_(p) and the sense emitter e_(s) are heated and the voltage V_(be)across the junction of the base and the sense emitter e_(s) decreases.However the reference emitter e_(r) is positioned at a remote positionthat would not be heated by the heat dissipated by the power emittere_(p) ; so that the voltage V_(be) across the junction of the base andthe reference emitter e_(r) remains relatively constant as the poweremitter e_(p) is heated. Thus as the temperature of the power emittere_(p) increases due to power dissipation in the power emitter e_(p) thevoltage difference between the sense emitter e_(s) and the referenceemitter e_(r) also increases. This voltage difference is proportional tothe temperature gradient between the power emitter e_(p) and the remoteposition 23 induced by the power dissipated in the power emitter e_(p).

A differential amplifier 32 has its input terminals respectively coupledto the sense emitter e_(s) and the reference emitter e_(r) for sensingthe voltage difference between the sense emitter e_(s) and the referenceemitter e_(r). The negative input terminal of the differential amplifier32 is connected to the sense emitter e_(s), and the positive inputterminal of the differential amplifier 32 is connected through a biasvoltage source E to the reference emitter e_(r). The output terminal ofthe operational amplifier 32 is connected through a diode D to the baseof the power transistor 30.

Drive current to the base of the power transistor 30 is provided from acurrent source I. The bias voltage source E has a predetermined voltagecorresponding to a predetermined temperature gradient indication level.The bias voltage source E biases the differential amplifier. When thevoltage difference between the sense emitter e_(s) and the referenceemitter e_(r) reaches this predetermined voltage, the output of thedifferential amplifier 32 becomes negative, thereby reducing the drivecurrent to the base of the power transistor 30 from the current sourceI. Accordingly the power provided to the power transistor 30 is reducedand the power dissipation in the power transistor is limited fromincreasing.

Since it is known at what temperature levels power transistors may bedestroyed, the predetermined bias voltage of the bias voltage source Eis selected to correspond to a temperature gradient indication levelthat enables the system of FIG. 6 to limit the power dissipation in thepower transistor 30 to less than a destructive level.

The system of FIG. 6 is the subject of applicant's co-pendingapplication filed on Sept. 16, 1977, application Ser. No. 833,736,cross-referenced hereinabove.

I claim:
 1. A power transistor comprisinga base; a power emitter in thebase; a sense emitter positioned in the base sufficiently close to thepower emitter for enabling the temperature of the power emitter to beindicated as a function of the voltage across the junction of the baseand the sense emitter; and a reference emitter positioned in the base ata remote position that would not be heated by the heat dissipated by thepower emitter for enabling the temperature at the remote position to beindicated as a function of voltage across the junction of the base andthe reference emitter; whereby a voltage difference proportional to thetemperature gradient between the power emitter and the remote positioninduced by power dissipated in the power emitter can be sensed betweenthe sense emitter and the reference emitter.
 2. A power transistoraccording to claim 1, wherein the sense emitter is distributed aroundthe power emitter for enabling a voltage difference proportional to themaximum instantaneous temperature gradient between the remote positionand any part of the periphery of the power emitter to be sensed.
 3. Apowet transistor according to claim 1, wherein a plurality of said poweremitters are contained in the base; and the sense emitter is positionedbetween the power emitters.
 4. A power transistor according to claim 3,wherein the sense emitter is distributed around and between the poweremitters for enabling a voltage difference proportional to the maximuminstantaneous temperature gradient between the remote position and anypart of the peripheries of the power emitters to be sensed.
 5. A powertransistor comprisingfirst and second bases; a power emitter in thefirst base; a sense emitter positioned in the first base sufficientlyclose to the power emitter for enabling the temperature of the poweremitter to be indicated as a function of the voltage across the junctionof the first base and the sense emitter; and a reference emitterpositioned in the second base at a remote position that would not beheated by the heat dissipated by the power emitter for enabling thetemperature at the remote position to be indicated as a function of thevoltage across the junction of the second base and the referenceemitter; whereby a voltage difference proportional to the temperaturegradient between the power emitter and the remote position induced bypower dissipated in the power emitter can be sensed between the senseemitter and the reference emitter.
 6. A power transistor according toclaim 5, wherein the sense emitter is distributed around the poweremitter for enabling a voltage difference proportional to the maximuminstantaneous temperature gradient between the remote position and anypart of the periphery of the power emitter to be sensed.
 7. A powertransistor according to claim 5, wherein a plurality of said poweremitters are contained in the first base; and the sense emitter ispositioned between the power emitters.
 8. A power transistor accordingto claim 7, wherein the sense emitter is distributed around and betweenthe power emitters for enabling a voltage difference proportional to themaximum instantaneous temperature gradient between the remote positionand any part of the peripheries of the power emitters to be sensed.